Plant biomarkers as early detection tools in stress management in food crops: a review

Main conclusion Plant Biomarkers are objective indicators of a plant’s cellular state in response to abiotic and biotic stress factors. They can be explored in crop breeding and engineering to produce stress-tolerant crop species. Abstract Global food production safely and sustainably remains a top priority to feed the ever-growing human population, expected to reach 10 billion by 2050. However, abiotic and biotic stress factors negatively impact food production systems, causing between 70 and 100% reduction in crop yield. Understanding the plant stress responses is critical for developing novel crops that can adapt better to various adverse environmental conditions. Using plant biomarkers as measurable indicators of a plant’s cellular response to external stimuli could serve as early warning signals to detect stresses before severe damage occurs. Plant biomarkers have received considerable attention in the last decade as pre-stress indicators for various economically important food crops. This review discusses some biomarkers associated with abiotic and biotic stress conditions and highlights their importance in developing stress-resilient crops. In addition, we highlighted some factors influencing the expression of biomarkers in crop plants under stress. The information presented in this review would educate plant researchers, breeders, and agronomists on the significance of plant biomarkers in stress biology research, which is essential for improving plant growth and yield toward sustainable food production.


Introduction
Food is essential to our daily lives and well-being because it provides energy to power all metabolic processes and nutrients for proper growth and disease resistance (Holder 2019).Food security refers to the condition in which people always have social, economic, and physical access to safe, nutritious, and sufficient food to meet their dietary requirements for a healthy life (Sadati et al. 2021).Ensuring adequate food security for the global human population, projected to expand to 10 billion by 2050, a 34% increase over the current population size, is a paramount global concern and imperative (Boretti and Rosa 2019).
One of the foremost strategies to attain global food security entails a substantial boost in food crop production.A recent study conducted by Galieni et al. (2021) underscores the urgency of this matter, revealing that, given the current population growth rate, food production must surge by approximately 70% to align with existing food demand.Between 2000 and 2019, the total primary crop production recorded a 54% increase, reaching 9.4 billion tonnes (FAO 2022).However, this positive trend does not extend uniformly to developing countries.In stark contrast, food production per capita in Africa has experienced a decline of about 5-13% over the past few decades, with approximately 73 million people suffering from severe food insecurity (Mohamed et al. 2021;Bjornlund et al. 2020).The principal causes of global food insecurity are abiotic and biotic stress factors.
Abiotic factors such as drought, salinity, heavy metal stress, flooding, and extreme temperatures significantly impact crop production and contribute to food insecurity in developed and developing countries (Summy et al. 2020).These stress factors endanger approximately 90% of arable lands, leading to a 70% reduction in major food crops (Waqas et al. 2019).For instance, drought was the primary cause of grain production shortages in the twenty-first century, with approximately one-third of global drought incidents occurring in Sub-Saharan Africa.Ethiopia and Kenya, in particular, endured some of the most severe drought periods in the past four decades (Kogan et al. 2019;Ofori et al. 2021).Furthermore, global temperature will rise by 2-4.9 °C by 2100, and approximately 5 million sites will experience heavy metal contamination at concentrations above regulatory limits (Raftery et al. 2017;Gonzalez Henao and Ghneim-Herrera 2021).
Biotic stress factors affect crop production and food security worldwide (Kaur et al. 2021).These factors, including bacteria, viruses, fungi, nematodes, weeds, and insects, are a huge constraint, destroying about one-third of agricultural produce valued at 750 billion US dollars annually (Mesterházy et al. 2020).According to the Food and Agriculture Organization (FAO) of the United Nations (UN), plant diseases alone incur global damages of 220 billion USD, while uncontrolled weeds could cause a 100% loss in crop yield annually in both developing and developed nations (He and Krainer 2020;Chauhan 2020).Biotic stress factors have historically played a role in some of the most severe famines.For example, in the United States, Puccinia graminis tritici fungi caused an epidemic that resulted in the loss of millions of bushels of wheat (Prasad et al. 2023).Additionally, the cassava mosaic disease epidemic in India, Sri Lanka, and Kenya has resulted in a yearly loss of approximately 25 million tons of cassava, which can lead to famine in subsequent years, especially in countries where it is a staple crop.These multifaceted challenges pose a significant threat to food security on a global scale.
Developing innovative methods and technologies to control or enhance plants' resistance to stress factors has become critical in improving crop growth and yield (Hareesh et al. 2023).An integral component of advancing these methodologies is gaining a profound understanding of plant response patterns to external influences (Galieni et al. 2021).Plants have a dynamic homeostasis system, enabling them to maintain a stable internal state, even amidst unpredictable external conditions.This equilibrium is crucial for their survival and optimal functionality (Torday 2015).
Plants synthesize biomarkers in response to stress to regulate cellular homeostasis.These biomarkers represent specific molecules or compounds that serve as measurable and quantifiable indicators of a plant's reaction to external stimuli (Steinfath et al. 2010).A diverse array of substances, including phytohormones, enzymes, proteins, and nucleic acids, constitute plant biomarkers, serving a pivotal role in monitoring and responding to changes in a plant's environment.Furthermore, they function as precursors, enabling the detection of potential stress well before it manifests as physical symptoms (Alharbi 2020).
Understanding plant physiology and developing strategies to improve crop resilience and productivity in changing environmental conditions.Studying plant biomarkers is an essential aspect of achieving this goal (Zhou et al. 2022).This review presents an overview of the typical cellular biomarkers expressed by plants in response to abiotic and biotic stress factors.It also discusses methods of identifying these biomarkers, their importance in crop engineering, and factors influencing their expression.The information provided in this review would enable agronomists and plant biotechnologists to develop rapid intervention mechanisms to improve crop resilience against both abiotic and biotic stress factors, thus contributing to global food security.

Plant biomarkers
Biomarkers have played a role in modern science for over half a century, but their significance has seen a noticeable increase since the twenty-first century.This surge can be attributed to new technological advancements that have made it possible to generate and validate biomarkers (Rapley and Whitehouse 2015).Biomarkers are indicators of the cellular state of an organism in response to environmental and biological factors (Paniagua-Michel and Olmos-Soto 2016).They are quantifiable and reproducible, and their concentrations differ significantly from those found in normal, unaffected organisms (Bodaghi et al. 2023).Plants, for instance, synthesize biomarkers in response to abiotic and biotic stress factors.These biomarkers function as early warning signals in plants, allowing the detection of stressors before they cause severe damage, often manifested as physical symptoms (Ernst 1999).
There are numerous laboratory-based techniques available for detecting and analyzing biomarkers in plant tissues.These methods involve examining either the biomarker itself or the genes that encode it (Pérez-Clemente et al. 2013).Examples of these techniques include Western blotting, MALDI-TOF, SDS-PAGE, 2D-GE, northern blotting, enzyme-linked immunosorbent assay (ELISA), LC-MS, and polymerase chain reaction (PCR) (Yang et al. 2021).In recent years, omic technologies have provided a more holistic understanding and aided in identifying plant biomarkers indicative of stress conditions.These include techniques such as genomics to identify significant stress-associated genes, proteomics to study variations in protein abundance relative to induced stress, metabolomics to study variations in cellular metabolites in response to stress, and transcriptomics to analyze gene expression patterns (Roychowdhury et al. 2023).Furthermore, the advancement of next-generation sequence approaches such as microarrays, RNA sequencing, and single-molecule real-time sequencing have provided high-throughput, sensitive and rapid methods of generating data from omic techniques (Saeed et al. 2022;Udawat 2023).
While plant biomarkers may not exhibit the same level of specificity as those in mammalian systems, they still play a significant role in detecting and mitigating plant stress factors (Steinfath et al. 2010).Given the increasing impact of abiotic and biotic stressors on plants, there is a growing global interest in using biomarkers at cellular and molecular levels to detect stress early, monitor changes in plant metabolism in response to stress, and prevent irreversible damage (Fernandez et al. 2016;Paes de Melo et al. 2022).This review will explore plant biomarkers with differential expression patterns under stress conditions.These biomarkers include abscisic acid, aquaporin, dehydrin, transcription factors, heat shock proteins, antioxidant enzymes, and sRNA.

Abscisic acid as a hormonal biomarker in plant stress responses
Abscisic acid (ABA) is a sesquiterpenoid with 15 carbon atoms synthesized from β-carotene via either the carotenoid pathway or the indirect pathway (mevalonic acid-independent pathway), as demonstrated in Fig. 1 (Hewage et al. 2020;Chen et al. 2020).
ABA plays a crucial role in the growth and development of plants, acting as an essential phytohormone (Chen et al. 2020).It is especially important in regulating several biochemical, molecular, and physiological processes in crop plants that are exposed to harsh abiotic stress conditions such as drought, salinity, extreme temperature, UV-radiation, and heavy metal stress (Vishwakarma et al. 2017).ABA is a key factor in synchronizing multiple processes that confer stress tolerance, such as root cell elongation, stomata closure, activating transcriptional and post-transcriptional stress defense responses, inducing the expression of stress-related genes, increasing hydraulic conductivity, and metabolic alterations (Muhammad Aslam et al. 2022).Numerous studies have found that an increase in cellular ABA concentrations is linked to adverse environmental conditions (Wang et al. 2021;Hu et al. 2022b;Yang et al. 2023b).For instance, under salt stress, rice plants accumulate ABA, which affects the growth and development of the root meristem (Huang et al. 2021).Similar examples of changes in the cellular concentration of ABA in crop plants under various abiotic stress conditions and their mode of action are reported in Table 1.
In contrast to abiotic stress, the accumulation of ABA during biotic stress can either increase plant tolerance or susceptibility (Gietler et al. 2020).The impact of ABA on plants is also influenced by pathogen type and biotic stress conditions (Bharath et al. 2021;Rasool 2022).High levels of cellular ABA can suppress the immune response of crop plants by repressing salicylic acid activity, making them more susceptible to biotrophic pathogens, such as Magnaporthe oryzae (in barley) and Botrytis cinerea (in tomato), which infect living host cells (Ulferts et al. 2015;Sivakumaran et al. 2016).On the other hand, the accumulation of ABA in crop plants under biotic stress causes stomata closure and increases callose deposits, which protects them from pathogen invasion (Hewage et al. 2020).Table 2 highlights additional examples of changes in cellular ABA concentration in response to biotic stress, including their mechanism of action.

Aquaporin as a biochemical marker in plant stress responses
Aquaporins (AQP) are transmembrane proteins that range in molecular weights from 23 to 31 kDa (Kapilan et al. 2018).They are found in various parts of plants, such as roots, leaves, seeds, flowers, and fruits, under normal physiological and stressful conditions (Hoai et al. 2020;Li et al. 2022)).Given the sedentary nature of plants, their numerous intracellular compartments, and the lack of a specialized circulatory system, there is a critical need for coordinated water regulation to adapt to multiple abiotic stresses, including salinity, drought, temperature, nutrient limitation, and heavy-metal toxicity (Banerjee and Roychoudhury 2020).
AQP serves as a channel for transporting and maintaining cellular water, ion, and neutral solutes, which explains their vital role in regulating some physiological and metabolic processes, such as root/leaf hydraulic conductivity, cell osmoregulation, transpiration, stomatal closure, cell regeneration, and cell elongation in plants (Zupin et al. 2017).Given the importance of AQP in plant cells, they are either upregulated or downregulated in response to stress.This modification of AQP abundance under different stressors helps to regulate osmotic balance (Kapilan et al. 2018).
AQP are typically classified into five subfamilies, namely tonoplast intrinsic protein (TIP), plasma membrane intrinsic proteins (PIP), small basic intrinsic proteins (SIP), X intrinsic proteins (XIP), and nodulin 26-like intrinsic proteins (NIP) (Zargar et al. 2017).These families of proteins perform specific roles within the cell during normal physiological conditions.The SIPs and some NIPs mediate the transportation of solvents within the endoplasmic reticulum, and the TIPs and NIPs mainly participate in the movement of minerals and organic micro-compounds due to their lower permeability to water molecules (Afzal et al. 2016).Conversely, the PIPs and TIPs mainly play a part in reactions during drought, cold, and salinity stress (Maurel et al. 2015).
Notably, plants' resistance to different stressors is directly proportional to the amount, distribution, and efficiency of aquaporins within the cells (Patel and Mishra 2021).For example, a study by (Lian et al. 2006), showed that 20% polyethene glycol (PEG)-induced water stress enhanced the accumulation of root and leaf plasma membrane intrinsic proteins in two rice cultivars, lowland rice (Oryza sativa L. cv.Xiushui 63) and upland rice (Oryza sativa L. cv.Zhonghan 3).Under drought stress, there was a consistent increase in the expression of the aquaporin gene (PIP1;5) in the leaves and roots of the more drought-tolerant pearl millet (Pennisetum glaucum (L.) R. Br) (Iwuala et al. 2020).Further examples of aquaporin expression under different abiotic stress conditions and their mode of action are shown in Table 3.

Dehydrin as a biochemical marker in plant stress responses
Dehydrins (dehydration-induced proteins) are a type of protein that are highly hydrophilic and thermostable, with molecular weights ranging from 22 to 60 kDa (Arumingtyas and Savitri 2013).These proteins belong to group 2 within the late embryogenesis abundant (LEA) family and are the most extensively studied of the seven groups of LEA proteins due to their crucial role in increasing plant tolerance to abiotic stress (Mertens et al. 2018).
During times of abiotic stress, plant organs accumulate dehydrins within their nucleus, mitochondria, cytoplasm, and membranes (Tiwari and Chakrabarty 2021).Due to their hydrophilic and thermostable properties, these proteins are able to maintain structural flexibility, even binding to membrane proteins during water deficit to prevent protein inactivation and coagulation (Liu et al. 2017).Furthermore, dehydrins' highly disordered and unstructured nature plays a crucial role in increasing tolerance to abiotic stress by preserving cellular integrity through the formation of hydrogen bonds within the cell membrane via coupled folding (Banerjee and Roychoudhury 2016).According to (Kalemba et al. 2015), dehydrin accumulation in various cellular compartments, organelles, and membranes in beech (Fagus sylvatica L.) seeds during development and storage prevents cellular damage caused by dehydration.Table 4 shows other examples of dehydrin expression in different crop plants under various abiotic stress conditions.
Dehydrins are grouped structurally into five sub-classes, namely SKn, Kn, YnKn, KnSYn, and YnSKn, based on the presence of conserved sequences (lysine-rich K-segment, unique to all Dehydrins; serine-rich S-segment; and tyrosine-rich Y-segment (Sun et al. 2021b).These conserved regions play essential roles in protecting plants from the adverse effects of osmotic stress.For example, the K-segment binds to cell membrane proteins, protecting them from electrolyte leakage and lipid oxidation.The S-segment is responsible for phosphorylation by the SNF1-related protein kinase, which influences the translocation of dehydrins from the cytosol to the nucleus and binding to calcium ions.However, the precise function of the Y-segment remains unknown (Murray and Graether 2022).Stival and colleagues reported the expression of dehydrin genes in Picea glauca in response to drought.They discovered that dehydrins with N1 K2 and N1 AESK2 sequences were the most receptive to the absence of water (Stival Sena et al. 2018).

Transcription Factors as molecular biomarkers in plant stress responses
As stated by Kabir et al. (2021), transcription factors (TFs) are multifunctional proteins that regulate various plant reactions to stress.They bind to transcription-factor binding sites in the promoter region of a DNA sequence, which then triggers a series of downstream reactions that result in the expression of target genes and the subsequent synthesis of functional proteins relative to stress (Wu et al. 2015).Stress signals perceived by cell wall and membrane receptors are transmitted to transcription factors through intracellular compounds such as reactive oxygen species (ROS), Ca 2+ , phosphatases and protein kinases.These TFs then regulate or stimulate the expression of responsive genes by binding to their respective cis-element (Shahzad et al. 2021).
Numerous families of transcription factors highly regulate plant defense gene expression in response to abiotic and biotic stress factors.These families include basic leucine zipper (bZIP), AP2/ERF, WRKY, NAC (NAM: no apical meristem, ATAF, CUC: cup-shaped cotyledon), droughtresponse elements binding proteins (DREB) and myeloblastoma (MYB) (Javed et al. 2020;Hrmova and Hussain 2021).Each of these transcription factor families comprises over 100 and can act as positive or negative regulators to enhance tolerance to the respective stress factor (Hu et al. 2022c).
Several research have reported changes in the expression of TF in crop plants in relation to abiotic stress.For example, (Xiang et al. 2008) reported an increased expression of a member of the bZIP transcription factor family (OsbZIP23) in drought-resistant upland rice genotype IRAT109 (Japonica) exposed to drought and salinity stress, as revealed through Northern-blot analysis.(Rahman et al. 2016) reported that the overexpression of finger millet (Eleusine coracana L.) transcription factor (NAC 67) in rice increased the tolerance of the resulting transgenic rice to drought stress by increasing the relative water content.Similarly, (Wei et al. 2019) demonstrated that overexpressing the GmWRKY54 transcription factor in soybeans conferred drought tolerance by activating target genes in the Ca 2+ and abscisic acid signalling pathway.Other examples of TF expression under different abiotic stress conditions and their mode of action are shown in Table 5.
TFs are also crucial in the plants' adaptation mechanism to biotic stress.For example, during a pathogenic attack, TF promotes the activation of pathogenesis-related protein genes and hypersensitive response, thereby increasing the plants' resilience to the pathogen (Campos et al. 2022).(Kaushal et al. 2021) also reported the upregulation of NAC, bHLH, and MYB transcription factors in banana cultivars resistant to Fusarium stress.Similarly, López et al. (2021) observed an increased resistance of a Columbia tomato cultivar to Fusarium oxysporum f. sp.lycopersici (Fol) infection due to the upregulation of WRKY transcription factor.Table 6 highlights more examples of TF expression under biotic stress.

Heat shock proteins as a biochemical biomarker in plant stress responses
Heat shock proteins (HSP), commonly known as stress proteins, are expressed by all living organisms, including plants and are widely distributed in cellular compartments and organelles such as the nucleus, endoplasmic reticulum, cytoplasm, and chloroplast (ul Haq et al. 2019;Singh et al. 2019).These proteins can be classified into five families based on their sequence homology and molecular weight: small Hsps (sHsp), Hsp60, Hsp70, Hsp90, and Hsp100 (Li and Liu 2019).Under normal physiological conditions, HSP constitutes between 5 and 10% of the total concentration of cellular proteins, where they play a highly significant role in regulating various growth and developmental processes such as controlling the cell cycle, assembling multi-protein units transporting into and out of subcellular compartments, and controlling protein degradation (Park and Seo 2015).However, their expression significantly increases under abiotic and biotic stress, a crucial adaptation to crop plants' stress tolerance (Hu et al. 2022a).Initially identified as proteins upregulated in plant cells under heat stress, it is now widely recognized that their expression also increases in response to other abiotic stress, such as heavy metal stress, drought, cold and UV radiation (Jacob et al. 2017).These stress factors induce changes in the physiological, cellular and metabolic function of the cell, leading to the aggregation, misfolding and dysfunction of native and non-native proteins (Mishra et al. 2018).HSPs function as molecular chaperones and perform several protective roles that safeguard the cell from the harmful effects of stress.For example, they buffer and bind to the hydrophobic regions of unfolded polypeptides during translation, preventing aggregation and amino-terminal misfolding and ensuring proper folding of the polypeptide chain (Roy et al. 2019).Additionally, they assist in stabilizing protein structure, maintaining normal conformation, and regulating cellular homeostasis (Khan et al. 2021).Using SDS-PAGE, isoelectric focusing (IEF), western blot, and dot blot techniques, Polenta et al. (2020) discovered an increased expression of HSP in tomatoes in response to extreme heat and cold conditions.Their findings highlight the importance and application of HSP as a plant stress biomarker (Polenta et al. 2020).Table 7 highlights some examples of HSP expression and their mechanism of conferring tolerance to plants under different abiotic stress conditions.
As observed in abiotic stress, HSPs are also integral components of the adaptive strategies employed by plants to mitigate the adverse effects of biotic stress.They enhance tolerance to biotic stress by regulating the stability and accumulation of various stress-responsive proteins, including pathogenesis-related (PR) proteins and antioxidant enzymes, thus detoxifying reactive oxygen species and preserving membrane stability.Numerous studies have documented the differential expression of heat shock proteins (HSP) in response to biotic stress.In a study conducted by Li et al. (2021), it was observed that the increased expression of HSP24 improved the resistance of grape berries (Vitis vinifera Cv 'Kyoho') against Botrytis cinerea infection.The researchers reported that this was due to the physical interaction of HSP24 with pathogenesis-related (PR) proteins, leading to the activation of the salicylic acid defense pathway against the fungi.Table 8 shows similar examples of HSP expression under different biotic stress conditions, including their mode of action.

Antioxidant enzymes as biochemical markers in plant stress responses
Enzymatic antioxidants are essential in promoting plant growth and development by counteracting the deteriorating effects of oxidative stress.They break down and eliminate free radicals produced in plant cells during biotic and abiotic stress (Saisanthosh et al. 2018).These antioxidant enzymes include superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), glutathione peroxidase (GuPx), and glutathione reductase (GR).SOD catalyzes the conversion of superoxide radicals to O 2 and H 2 O 2 , CAT converts two molecules of H 2 O 2 into water and O 2 , and POX scavenges H 2 O 2 within extracellular spaces.In addition, APX utilizes ascorbic acid to reduce H 2 O 2 to water, GPX catalyzes the breakdown of H 2 O 2 and GR catalyzes the conversion of oxidized glutathione (dimeric GSSG) to reduced glutathione (monomeric GSH) (Rajput et al. 2021;Kapoor et al. 2020).
Abiotic stress triggers physiological and metabolic changes such as stomatal closure, reduced CO 2 availability, and disruption of photosynthetic enzymes and photosystems (Sachdev et al. 2021).These stress-induced changes causes the accumulation of free radicals and reactive oxygen species such as singlet oxygen, superoxide ion, and hydrogen peroxide in various plant tissues, leading to oxidative damage and cell death (Dumanović et al. 2021).In response to increased ROS production, plants upregulate the synthesis of antioxidant enzymes to scavenge and maintain cellular ROS homeostasis, thereby mitigating the adverse effects of abiotic stress (Huang et al. 2019).Numerous studies have shown that under various abiotic stressors, plants upregulate antioxidant enzymes.Table 9 highlights a few of these.
Biotic stressors, such as pathogenic infections and wounding, trigger specific plant defence responses.This response involves generating elevated levels of reactive oxygen species (ROS), also known as oxidative burst, to prevent pathogen invasion and proliferation and facilitate death (Ali et al. 2018).ROS speeds up cell regeneration and wound healing by preventing pathogen invasion at the injury site (Polaka et al. 2022).Furthermore, ROS acts as a signalling molecule and regulates several signalling pathways involving cell wall modification, changes in gene expression and hypersensitive response (HR), further protecting plants from biotic stress (Lehmann et al. 2015).
Nevertheless, excess production of ROS beyond a specific concentration threshold disrupts cellular homeostasis, resulting in protein peroxidation, enzyme inhibition, breakdown of cellular components, DNA fragmentation, activation of apoptosis, and cell death (Wang et al. 2019b).Plants deploy antioxidant enzymes as the foremost defense line to counteract these detrimental effects.These enzymes play a crucial role in protecting plants from the harmful consequences of ROS generated by biotic stress factors (Sahu et al. 2022).Small RNA as a molecular biomarker in plant stress responses Plant Small RNAs (sRNA) constitute a category of noncoding ribonucleic acid molecules spanning from 21 to 24 nucleotides in length (Morgado and Johannes 2019).They are ubiquitously distributed across diverse cell types and tissues, actively participating in various biological processes, including plant reproduction, growth, and response to biotic and abiotic stressors (Zhan and Meyers 2023;González Plaza 2020).Plant small RNAs (sRNAs) can be classified into several categories based on their biogenesis, including microRNAs (miRNAs), piwi interacting RNAs (piRNAs), small interfering RNAs (siRNAs), small nuclear RNAs (snRNAs), and small nucleolar RNAs (snoRNAs) (Brant and Budak 2018).However, miRNAs and siRNAs are the most widely studied, primarily due to their pivotal roles in enhancing plant resilience against abiotic and biotic stress factors (Chen et al. 2018).miRNAs and siRNAs are both generated from doublestranded RNAs (dsRNAs) in a series of downstream reactions involving RNA polymerase and Dicer-like (DCL) proteins (Mahto et al. 2020).However, while DCL1 trims miRNA, siRNA is generated from multiple pathways involving diverse exogenous and endogenous dsRNAs precursors by DCL2-4 proteins.The generated miRNAs and siRNAs are then incorporated into Argonaute (AGO) proteins to form the RNA-induced silencing complex (RISC).This complex regulates target genes at the transcription and post-transcriptional levels (Tang et al. 2021).
The mechanism of action of sRNA at target sites involves transcriptional and post-transcriptional gene silencing through DNA methylation, RNA slicing, histone modification, and translational repression (Tang et al. 2022;Patel et al. 2020).Additionally, they exhibit diverse regulatory patterns in response to varying stress conditions, with upregulation observed in positive regulators and downregulation in negative stress regulators (Sun et al. 2021a).While specific sRNAs are conserved, overseeing shared traits across plant species, others are specific to particular species.Both species-specific and conserved sRNAs play a pivotal role in plant stress responses and can be used as plant biomarkers (Jyothsna and Alagu 2022).
sRNA acts as a modulator in response to diverse abiotic stress conditions.They regulate the upregulation or downregulation of target genes in stress-associated pathways at both the transcriptional and post-transcriptional levels (Mondal et al. 2023).For instance, miRNA contributes to enhanced drought tolerance by regulating the expression of drought-responsive genes, transcription factors, and other biomolecules, including proline, dehydrin, and LEA proteins (Saroha et al. 2017).Furthermore, miRNA regulates salinity stress tolerance by modulating ion homeostasis and hormone signalling pathways (Banerjee et al. 2017).More examples of some sRNA identified in various crop plants under abiotic stress, their identification procedures, and their mode of action are highlighted in Table 11.
In addition, small RNA (sRNA) enhances plant tolerance to biotic stress by targeting genes involved in regulating multiple plant immune responses, including pathogen-associated molecular pattern(PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) (Brant and Budak 2018).
Both PTI and ETI work synergistically to induce various defense mechanisms, such as the accumulation of salicylic acid (SA), callose deposition on the cell wall, hypersensitive response (HR), generation of reactive oxygen species, expression of pathogenesis-related (PR) genes, and cell death at the infection site (Tang et al. 2021).For example, In barley (Hordeum vulgare L.), miRNA was upregulated by the infection of Blumeria graminis f. sp.hordei, a fungus that causes powdery mildew disease.Further experiments suggest that increased miRNA expression confers tolerance by activating a cascade of reactions, leading to disease resistance and cell death signalling (Liu et al. 2014).
Table 12 highlights examples of some sRNA identified in various crop plants in response to biotic stress, their identification procedures, and their mode of action.

Structure-activity relationship of biomarkers
Understanding how biomolecules are structured is essential in determining their role in plants under various stress conditions.This is referred to as structure-activity relationships (SAR), which explores the relationship between a molecule's biological activity and its three-dimensional (3D) structure.
Understanding the structure and c functional groups of a plant stress biomarker aids in predicting physiological and biochemical function (Šamec et al. 2021).For example, aquaporin is a transmembrane protein with unique characteristics that allow it to transport water in plants during osmotic stress (Wang et al. 2020).Aquaporin comprises 6 segments of alpha-helical hydrophobic protein domains and several NPA (asparagine-proline-alanine).The presence of hydrophobic segments and the formation of hydrogen bonds between water molecules and polypeptide residues enables rapid water transport within plant tissues (Adeoye et al. 2021).Similar examples showcasing the structure-activity relationship of various biomarkers can be found in Table 13.

Application of plant biomarkers in crop engineering
To achieve sustainable agriculture and produce enough food for the world's growing population, effective strategies for dealing with extreme conditions such as temperature extremes, pathogen attacks, herbivores, drought, salinity, and heavy metal stress are required (López-Arredondo et al. 2015).To achieve this, it is important to understand the cellular, epigenetic, and molecular mechanisms that orchestrate plant response to various biotic and abiotic stressors.This will lay the groundwork for engineering crops with faster growth rates, higher yield, and productivity (Jiménez Bremont et al. 2013).Modern crop science research has been transformed by the understanding of omic technologies (metabolomics, genomics, proteomics, and transcriptomics), which allow for more robust studies on the primary metabolites, proteins, genes, and molecular networking pathways associated with plant responses to various abiotic and biotic stresses (Yuan et al. 2008).The findings of these studies have aided in the identification of biomarkers that confer resilience to an adverse stress factor, as well as in the introduction of these desired characteristics into model crops and various economically important crops such as barley, maize, wheat, and rice, among others (Yang et al. 2021).These biomarkers are critical in developing new crop varieties (transgenic crops) that are more resistant to adverse environmental conditions (Rodziewicz et al. 2014).Plant biomarkers are widely used in genetic crop engineering to create transgenic crops with higher yields under adverse abiotic and biotic conditions (Leetanasaksakul et al. 2022;Bakhsh and Hussain 2015).Transgenic crops have proven to be a complementary and effective alternative in modern agriculture, increasing yield by 22%, reducing pesticide use by 37%, and increasing profit by 68%.These crops are grown on approximately 180 million hectares worldwide (James 2014).
Researchers have discovered that overexpressing the barley dehydrin gene (HVA1) in wheat can lead to the development of transgenic wheat that is better adapted to salinity and drought stress.The transgenic wheat plants displayed improved membrane stability and reduced electrolyte leakage, according to research by (Habib et al. 2022).The NAC transcription factor has been identified as a critical TF that enhances the resilience of cowpeas (Vigna unguiculata L. Walp.) to a range of environmental stresses, including drought, heat, cold, and salinity.By overexpressing two native cowpea NAC genes (VuNAC1 and VuNAC2), significant improvements in tolerance to these stressors were achieved.The resulting transgenic plants displayed enhanced antioxidant activity, membrane integrity, water use efficiency, and Na + /K + balance.These improvements culminated in an estimated threefold increase in growth and yield, (Srivastava et al. 2023).

Application of plant biomarkers in crop breeding
When faced with challenging environmental or biological circumstances, plants may demonstrate alterations that involve either the activation or suppression of specific biomarkers, including transcription factors, enzymes, osmolytes, hormones, and small RNA (Isah 2019).These biomolecules are known to prevent the destruction of cellular components and restore homeostasis, which is critical for plant growth and development under stress conditions (Ben Rejeb et al. 2014).Figure 2 illustrates some biomarkers expressed by plants under stress conditions.Comparing the expression of these biomarkers provides information about the tolerance level of the plants and is also helpful for studying and analyzing different plant genotypes, species, and cultivars (Chaudhary et al. 2020).The knowledge of plant biomarkers has been used in crop breeding to identify phenotypes or cultivars that are more resilient or susceptible to different abiotic and biotic stress factors and has aided in

Factors influencing the expression of plant biomarkers in crop plants under stress
Plant biomarkers are influenced by several factors including tissue or organ specificity, circadian readings, developmental stage, species, and cultivars, which cause differential expression patterns in crops exposed to the same stress conditions (Fernandez et al. 2016).
Under drought stress, two aquaporin proteins, PIPs and TIPs, were differentially expressed in Brassica napus plant tissues.PIPs and TIPs were downregulated in the root but upregulated in the leaves.Reduced AOP expression in the roots may be associated with the need to prevent water loss from the root due to water deficit.In contrast, increased leaf AQP expression enhances water transportation to regulate normal metabolic functions within the plant (Sonah et al. 2017).Similarly, Yu et al. (2019) observed variations in the expression of HSP in the leaves and roots of cassava plants exposed to drought stress.The leaves showed greater levels of upregulated HSP genes compared to the roots.Transcription factors are other biomarkers with variable expression patterns in different plant tissues.Using qRT-PCR, the expression profile of a specific wheat transcription factor, MYB4, in response to salinity stress, was investigated.The findings revealed increased expression levels in the shoots, whereas a simultaneous reduction in expression was observed in the roots (Sukumaran et al. 2023).Furthermore, the exposure of pepper plants (Capsicum annuum L.) to pathogenic infection resulted in variations in the expression profile of antioxidant enzymes in both the roots and leaves (Zheng et al. 2004).
Data from several research studies has shown that plant biomarkers are differentially expressed at different stages of plant growth when exposed to the same stress condition.Castañeda-Saucedo et al. (2014) reported differences in dehydrin accumulation at the seed filling and pod formation

Dehydrin
The presence of a distinctive lysine-rich conserved region known as the K-segment can form an amphipathic helix and bind to macromolecules to prevent stress-induced damage The presence of numerous charged and polar amino such as Ser, Gln, Pro, Lys, Glu, Ala, and Gly confers Antioxidant and metal chelating properties Smith and Graether (2022), Rorat (2006) Transcription factor A DNA-binding domain aids the transcription factors in binding specifically to the cis-acting element in the promoter region of stress-induced genes An activation domain triggers downstream reactions that lead to the activation or repression of the gene Kimotho et al. (2019) Superoxide dismutase Metal ions (Cu, Zn, Mn, and Fe) between the two sub-units act as cofactors in SOD, enhancing its catalytic activity of scavenging toxic metabolites by donating electrons to ROS Stephenie et al. (2020) Ascorbate peroxidase The enzyme contains amino acid residues that boost its activity, such as lysine, cysteine, and arginine, which form hydrogen bonds with ascorbate/ substrate and histamine, which aids the cleavage of the oxygen-oxygen bond in hydrogen peroxide Iron in the heme prosthetic group increases the enzyme's catalytic activity Dąbrowska et al. (2007)

Catalase
The active site comprises a heme group with three amino acid residues: tyrosine at the proximal end, histidine, and asparagine at the distal end, all of which are crucial for its catalytic activity Karakus (2020) stages of common beans (Phaseolus vulgaris L.) subjected to drought stress.In a similar experiment, Samarah and colleagues observed a more significant dehydrin accumulation in soybean seeds (Glycine max L.) at the maturity stage compared to the developmental stage (Samarah et al. 2006).Furthermore, high-throughput techniques, such as gene profiling and RNA sequencing, were employed to examine the expression of the transcription factor (bZIP) in wheat (Triticum aestium L.) plants exposed to heat stress.The investigation revealed variations in expression across various developmental stages, with the highest expression level observed at day five post-anthesis (Agarwal et al. 2019).Ge and colleagues studied the expression of aquaporin at different stages of germination in Brassica napus plants subjected to cold, salinity and drought stress.The findings revealed an up-regulation of the AQP genes at the germination and early seedling stage but downregulated at the maturity stage (Ge et al. 2014).
The time of the day or circadian changes have also been reported to influence the expression of biomarkers in plants under stress conditions.For instance, the expression of Aquaporin (PIP) was upregulated at dawn and downregulated at dusk in different plants, which may be due to an increase in the rate of transpiration during the day (Hachez et al. 2012;Heinen et al. 2014;Ding et al. 2020).In a recent study conducted by Lu et al. (2021), it was discovered that while cold stress induces the activity of transcription factors (OsDREB1B and OsDREB1C) throughout the day, peak levels were observed during daytime as opposed to nighttime.Similarly, in peach plants (Prunus persica L.), cold stress leads to a more significant induction in the activity of dehydrins (DHN 1 and 3) in the morning (Artlip et al. 2013).In addition, transcriptomic analysis revealed that heat stress response genes such as HSP, antioxidant enzymes and specific TFs are more induced by heat stress in the morning and early afternoon than at other times of the day (Bonnot et al. 2021;Blair et al. 2019;Lai et al. 2012).Abscisic acid is another plant stress hormone controlled by circadian changes, with peak Fig. 2 Schematic representation of plant response pattern to stress and expression of biomarkers (SOD-superoxide dismutase; CAT-catalase; APX-ascorbate peroxidase, POX-peroxidase; GPX-glutathione peroxidase and GR-glutathione reductase; HSP-heat shock proteins levels observed at specific times during the day in different crop plants (Hotta et al. 2013;Khan et al. 2010).
Cultivar or plant species is another factor influencing the expression of biomarkers subjected to the same stress conditions.Several examples of this have been documented in literature.Under salinity stress, two rice cultivars (Cotaxtla and Tres Ríos) exhibited different NAC transcription factor expression patterns (García-Morales et al. 2014).Variations in the expression profile of dehydrin between two grape species (V.vinifera and V. yeshanensis) in response to drought were also reported by Yang et al. (2012).While there was an upregulation of dehydrin genes in the V. yeshanensis species between 1 and 2 days post-drought imposition, a response was only observed in the other species between 2 and 3 days post-drought treatment.The results of the field survey conducted by Oliveria and colleagues revealed that differential expression of antioxidant enzymes was observed among ten different cultivars of cowpea (Vigna unguiculata L.) under nematode infestation (Meloidogyne incognita) (Oliveira et al. 2012).In addition, transcriptome analysis has revealed variations in the expression of the transcription factors (bHLH family, AP2-ERF, MYB and WRKY) in the cultivars of potato, tomato and spinach when subjected to pathogen attack (Bayoumi et al. 2021;Kandel et al. 2020;Upadhyay et al. 2016).

Conclusion
Abiotic and biotic stressors pose severe challenges to global food security, rendering current crop yield insufficient to meet future global food demand.These stressors have been documented as significant contributors to a substantial decline in crop yields, affecting developed and developing nations.Therefore, we need to focus on developing innovative technologies and methods to produce stress-tolerant, widely-adapted, and high-yielding crops under various stress conditions.One way to achieve this is by understanding plant response patterns to external factors.When exposed to an environmental or biological stress factor, some biomolecules are up or downregulated in plants.Such biomolecules can function as effective plant biomarkers to monitor plant stress response.In this review, we discussed common biomarkers expressed by plants during abiotic and biotic stress conditions and the relationship between their biological activity and three-dimensional (3D) structures.Plant stress biomarkers have a wide range of applications in crop breeding and crop engineering in producing stress-resilient crops with higher yields under adverse conditions.In addition, they can be used for identifying plant cultivars, species, and genotypes with the desired or improved traits.Considering the potential of plant stress biomarkers, more research on the mechanism underlining plant responses to various stress factors and intensive development of analytical platforms and databases are encouraged to standardize plant stress biomarkers for use in breeding novel stress-resistant crop varieties with better yield.

Fungi
f.sp.lini) Linum usitatissimum L. ABA Immunoassay kit Accumulation of ABA inhibits pathogen invasion by stimulating stomata closure, inducing expression of defense genes, and facilitating callose deposition on plant cell walls Boba et al. (2020) liquid chromatography-mass spectrometry Elevated levels of ABA enhance callus synthesis within the plasmodesmata and suppress the activity of the callose-degrading enzyme β-1,3-glucanase, thus preventing the intrusion of viruses into host plant cells.ofABA in infected plant tissues enhances resistance to nematode attacks by acting as a signalling hormone that triggers the activation of the systemic defense pathway in the host plant Family: SK3 (DHN 1) LOC100816147 Accumulation of dehydrin proteins increases the resilience of the droughttolerant cultivar by facilitating membrane stability, ion flow, and water retentionArumingtyas and Savitri (2013) Cold, drought and salinity Triticum aestivum

Table 1
Abscisic acid as a hormonal biomarker in plants' adaptive response to various Abiotic stress

Table 2
Abscisic acid as a hormonal biomarker in plants' adaptive response to various biotic stress

Table 4
Dehydrin, a biochemical marker in plants' adaptive response to various abiotic stress Table 10 highlights examples of enzymatic antioxidants expressed in response to biotic stress factors and their mechanism of action.

Table 8
Heat shock protein as a protein biomarker in plants' adaptive response to various biotic stress

Table 9
Antioxidant enzymes as biochemical markers in plants' adaptive response to various abiotic stress

Table 13
Relationship between a biomarker's biological activity and its three-dimensional (3D) structure